enable js to listen on an existing server

This patch enables JavaScript to pass a server object to `yoke.listen()`.

The method to check if the first parameter to `listen()` is a server object is declared private in `io.vertx~lang-rhino~2.0.0-CR1/vertx/http.js` so we maybe have to bug upstream at some point in the future if the method disappears for some reason... ;)